A significant impediment to the construction of efficient mini-lasers and amplifiers and certain fiber lasers and amplifiers is the functional requirement that the gain materials have a high absorption coefficient at the pump wavelength. Increasing the concentration of the lasing ion to increase this absorption is usually limited by concentration quenching and ion clustering effects and/or by the solubility limit of the lasing ion in the gain material. In silica, for example, cooperative up-conversion and ion clustering effects occur at high dopant concentrations and can depopulate the metastable level of the active ion. Therefore, increased doping in a silica glass may improve the absorption, but at the expense of the gain. Other glasses, such as commercial phosphate laser glasses exhibit high solubility and large emission cross section for many rare-earth ions without severe concentration quenching and ion clustering effects, thereby permitting much larger dopant concentrations and thus much higher absorptions without sacrificing gain properties. In any case, even in these phosphate glasses, laser ion dopant levels greater than about 10 weight percent suffer from a sharp decrease in gain, thereby severely limiting the absorption length available at the relevant pump radiation wavelength.
Attempts to address this issue in the past have included the use of a more heavily doped sensitizer in conjunction with the low-doped lasing ion. The sensitizer absorbs the pump energy and then transfers that energy to the lasing ion. For example, in U.S. Pat. No. 4,962,067, “Erbium Laser Glass Compositions”, the present inventor describes a phosphate glass composition, which uses ytterbium as a sensitizer ion and erbium as the lasing ion. In the '067 patent, the amount of ytterbium is limited by the solubility of ytterbium in the base glass, which appears to be about 8 mole percent Yb2O3. As is typical in the formation of laser glass the base glass is initially formed and then the sensitizing agents such as ytterbium and lasing ion(s) are added to the base glass. A typical base glass comprises: 50 to 65 mole % phosphate pentaoxide; 10 to 30 mole % of R2O, wherein R is selected from the group Li, Na, K, Rb, Cs, or a mixture thereof; 5 to 25 mole % of MO, wherein M is selected from the group Mg, Ca, Sr, Ba, Zn, and mixtures thereof. The base glass may also include up to 10 mole % of Al2O3. Attempts to increase the amount of ytterbium in this base glass beyond this amount results in devitrification of the melt and the formation of crystals within the resulting glass. That dopant limitation of about 8 mole percent of ytterbium oxide in the base glass results in a requirement of approximately a 5 millimeter path length of material to absorb 90% of the diode pump energy in the 910 to 970-nanometer range, the useful range for ytterbium-sensitized erbium doped laser glass. For many mini-laser, fiber laser and fiber amplifier applications, 5 millimeters is too long of an absorption length for efficient operation. In addition, applications requiring the side-pumping of fiber lasers and/or end-pumping of double clad fiber lasers and amplifiers also need significant reductions in absorption lengths to become more efficient. Similar limitations are shown in U.S. Pat. No. 6,611,372 wherein the ytterbium in the phosphate glass is at 12 weight % with is approximately 7 mole percent.
In summary, there is a need for a lasing glass that can accommodate larger quantities of sensitizing agents such as ytterbium to raise the output of lasers doped with lasing ions such as erbium, neodymium, holmium, and thulium.